CVE-2026-46540 Details

A vulnerability exists in the Nimiq light blockchain implementation of the Proof-of-Stake protocol, prior to version 1.4.0. The issue arises in the LightBlockchain::rebranch() function when it adopts a fork chain whose tip is a macro block (checkpoint or election). The function only updates the self.head parameter, neglecting to update self.macro_head, self.election_head, self.current_validators, or store the election header in the chain_store. This behavior contradicts the full Blockchain::rebranch() function, which correctly updates all macro and election states when the new head is a macro block. As a result, after a rebranch to a macro block, the outdated macro_head causes subsequent macro blocks to be verified against the incorrect predecessor, disrupting the verification process. If the rebranch target was an election block, the stale current_validators parameter causes all subsequent blocks to fail the validation process, completely halting the light client's chain progression.

Users can upgrade to Nimiq version 1.4.0 or later, where this vulnerability has been patched.